Mass spectrometry (MS) is used widely for identifying and quantifying molecular species in a sample. During analysis, molecules from the sample are ionized to form ions. A detector produces a signal relating to the mass of the molecule and charge carried on the molecule and a mass-to-charge ratio (m/z) for each of the ions is determined.
A chromatographic separation technique may be performed prior to injecting the sample into a mass spectrometer. Chromatography is a technique for separating compounds, such as those held in solution, where the compounds will exhibit different affinity for a separation medium in contact with the solution. As the solution flows through such an immobile medium, the compounds separate from one another. Common chromatographic separation instruments include gas chromatographs (GC) and liquid chromatographs (LC). When coupled to a mass spectrometer, the resulting systems are referred to as GC/MS or LC/MS systems. GC/MS or LC/MS systems are typically on-line systems in which the output of the GC or LC is coupled directly to the MS.
In an LC/MS system, a sample is injected into the liquid chromatograph at a particular time. The liquid chromatograph causes the sample to elute over time resulting in an eluent that exits the liquid chromatograph. The eluent exiting the liquid chromatograph is continuously introduced into the ionization source of the mass spectrometer. As the separation progresses, the composition of the mass spectrum generated by the MS evolves and reflects the changing composition of the eluent.
Mass spectral data is typically generated by the impact of ions on the one or more ion detectors of the MS instrument, which provide signals which can be processed to provide information as to the mass to charge (m/z) ratios and the number of ions (e.g. by the intensity of the ion count) at a particular m/z, the information typically being provided in the form of a mass spectrum. Mass spectra may be further analysed to elucidate structural information about the compounds analysed.
Typically, at regularly spaced time intervals, a computer-based system samples and records the spectrum. The response (or intensity) of an ion is the height or area of the peak as may be seen in the spectrum. The spectra generated by conventional LC/MS systems may be further analyzed. Mass or mass-to-charge (m/z) ratio estimates for an ion are derived through examination of a spectrum that contains the ion. Chromatographic retention time estimates for an ion are derived by examination of a chromatogram that contains the ion.
Two stages of mass analysis (MS/MS also referred to as tandem mass spectrometry) may also be performed. One particular mode of MS/MS is known as product ion scanning where parent or precursor ions of a particular m/z value are selected in the first stage of mass analysis by a first mass filter/analyzer. The selected precursor ions are then passed to a collision cell where they are fragmented to produce product or fragment ions. The product or fragment ions are then mass analyzed by a second mass filter/analyzer.
In connection with sample analysis, ion mobility spectrometry (IMS) may be used in combination with MS. The foregoing may be denoted as IMS-MS or IMS/MS for the combination of ion mobility spectrometry and mass spectrometry. Such an analyzer included in a system performing IMS/MS may include first separating ions according to their mobilities (IMS) followed by the mass analyzer separating ions according to their mass to charge (m/z) ratio (e.g., followed by MS). Chromatographic separation, such as by LC or GC, may be performed prior to and coupled with IMS/MS. Such experiments using LC and GC may be, respectively, be referred to as performing LC/IMS/MS and GC/IMS/MS.